The long term goal of the proposed research is to understand the molecular basis of disease resistance in plants. Plants are able to specifically detect pathogens and actively respond. It is not understood how this detection is accomplished or how detection is translated into a resistant response. In many plant-pathogen interactions, disease resistance is determined by a single plant "resistance" gene that is in some way specific to a single pathogen "avirulence gene"; loss of either member of this gene pair results in plant disease. To investigate the basis of such "gene- fore-gene" interactions, we have developed a model system that uses the small mustard Arabidopsis thaliana as a host plant and the bacterial plant pathogens Pseudomonas syringae and Xanthomonas campestris as pathogens. These organisms are ideally suited to a molecular genetic approach due to the rapidity with which genes of these organisms can be identified and cloned and then reintroduced into their respective genomes. In preliminary studies we have identified an avirulence gene from Pseudomonas syringae that determines avirulence on specific ecotypes of A. thaliana. The primary objective of the proposed research is to identify plant genes that are required for the expression of disease resistance; these genes include specific resistance genes that "interact" with cloned avirulence genes, possible signal transduction genes, and genes that directly control physiological responses. The following experiments will be performed to accomplish this objective: 1. Avirulence genes from P. syringae and X. campestris will be identified and cloned. These genes will be used to identify corresponding resistance genes in A. thaliana. 2. A. thaliana resistance genes corresponding to the avirulence genes identified in the first set of experiments will be genetically mapped. 3. Mutants of A. thaliana will be identified that are susceptible to infection by bacterial pathogens expressing cloned avirulence genes. 4. Mutants will be characterized genetically and molecularly. Each mutant will be tested against our collection of avirulence genes to determine if the mutation is specific to a single avirulence gene or it is affects resistance corresponding to several avirulence genes. Allelism of mutations to genetically mapped resistance genes and to the other mutations will be determined. 5. Based on the above analyses, specific disease resistance genes and signal transduction genes (if identified) will be chosen for further characterization by molecular cloning. These experiments will all significantly to our understanding of how the interaction of plant disease resistance genes and pathogen avirulence genes leads to disease resistance. This interaction encompasses cell:cell recognition, signal transduction, and gene activation, all of which are important and basic biological processes.